The Neptune File: A Story of Astronomical Rivalry and the Pioneers of Planet Hunting

The Neptune File: A Story of Astronomical Rivalry and the Pioneers of Planet Hunting

by Tom Standage
     
 

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The Neptune File is the first full account of the dramatic events surrounding the eighth planet's discovery, and the story of two remarkable men who were able to "see" on paper what astronomers looking through telescopes for more than 200 years had overlooked." "Drawing on long-lost documents in George Airy's Neptune scrapbook, which resurfaced mysteriously at an…  See more details below

Overview

The Neptune File is the first full account of the dramatic events surrounding the eighth planet's discovery, and the story of two remarkable men who were able to "see" on paper what astronomers looking through telescopes for more than 200 years had overlooked." "Drawing on long-lost documents in George Airy's Neptune scrapbook, which resurfaced mysteriously at an observatory in Chile in 1999, The Neptune File is an exploration of the science that underpins planetary astronomy. And the tale continues to unfold, as Tom Standage relates: Since 1995, astronomers have discovered more than 40 planets outside our solar system, opening an intriguing window on the universe. Yet none of these planets have ever been seen. Their discovery - and the history of science - owes much to the two men who unlocked the secret to locating unseen new worlds.

Editorial Reviews

bn.com
From the author of The Victorian Internet comes another fascinating excursion into the history of science to show the past's continuing relevance. The mathematical techniques developed in the 19th century to discover the planet Neptune laid the basis for modern planet hunting around distant stars.

Product Details

ISBN-13:
9780425181737
Publisher:
Penguin Publishing Group
Publication date:
11/01/2001
Edition description:
REISSUE
Pages:
256
Product dimensions:
5.62(w) x 7.46(h) x 0.68(d)

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Chapter One


The Musician of the Spheres


Then felt I like some watcher of the skies
When a new planet swims into his ken.

—John Keats
   "On First Looking into Chapman's Homer"


Sometime between ten and eleven o'clock on the night of Tuesday, March 13, 1781, William Herschel was looking at the stars through a homemade telescope from his garden in the English spa town of Bath. Herschel was a musician by trade, but his passion for astronomy had grown over the previous few years to the point that he was spending more time with his astronomical instruments than with his musical ones. What he saw through his telescope that night was to change his life completely and win him widespread and lasting fame. He was about to become the first person to discover a new world.

    For centuries, astronomers had followed the five "classical" planets—Mercury, Venus, Mars, Jupiter, and Saturn—across the skies. The existence of these planets, which resemble bright stars clearly visible to the naked eye, has been known to humankind almost from the beginning of civilization itself. But since antiquity nobody had ever discovered any additional planets, and the idea that there might be more such objects lurking in the heavens seemed outlandish.

    Herschel was in a unique position to make such a fortuitous discovery. Having taught himselfastronomy, he had little interest in the tedious business of measuring the positions of the stars or working out tables of the positions of the Moon and planets, which was what professional astronomers spent most of their time doing. Instead, as an amateur, he was free to roam the skies at will, looking at whatever took his fancy. At the same time, Herschel was no ordinary amateur. As a result of the trial-and-error process of learning to build his own telescopes, he had, without realizing it, become the finest telescope maker in the world.

    The telescope he was looking through on that chilly March night was one of his favorites: 7 feet long, 7 inches in diameter, with a wooden tube and a handmade mirror that was the result of hours of painstaking grinding and polishing. The tube was supported by an elaborate wooden frame, with a system of cords and pulleys and three small crank-handles to adjust its position. Herschel also had beside him a set of his own eyepieces, each one mounted in a tube of cocus wood, the kind of wood used in the body of an oboe—one of the first musical instruments he had learned to play as a boy. By removing one eyepiece and inserting another, Herschel could vary the magnifying power of his telescope.

    Increasing the magnification would, for example, make a planet (such as Jupiter or Saturn) appear larger and more distinct. Herschel particularly enjoyed looking at Saturn, which, with its spectacular ring system, is a magnificent sight in even the smallest, feeblest telescope. But on that particular night he was looking at stars, not planets, using one of his less powerful eyepieces, with a magnifying power of 227 times.

    As he swept the telescope over the constellation of Gemini, Herschel noticed something unusual. He decided to take a closer look and removed the telescope's eyepiece in order to substitute a more powerful one. Switching to the eyepiece with a magnification of 460, he found that his mystery object appeared twice as large as it had under a magnification of 227; when he switched to the eyepiece with a magnification of 932, it doubled in size again. Because stars are so distant that they appear as points of light, no matter how great the magnification, this meant the mystery object was definitely not a star. So he noted it in his astronomical journal as "a curious either nebulous star or perhaps a comet."

    The object was essentially a slightly fuzzy blob. Herschel knew that a fuzzy blob could be one of two things: a nebula (a generic term covering all manner of star clouds, clusters, and what we now know are distant galaxies) or a comet (an orbiting snowball within our own solar system that brightens and spews out a tail of gas and dust as it approaches the Sun). The two can be told apart by seeing whether or not they move relative to the fixed stars. Nebulae, like stars, stay fixed; comets, like planets, move from one night to the next. Hoping that perhaps he had discovered a comet, Herschel noted its position so he could observe it after a few days and see if it was still in the same place. A few days later, on the night of Saturday March 17, he noted in his journal, "I looked for the comet or nebulous star, and found that it is a comet, for it has changed its place."

    To have discovered a comet was quite an achievement, and Herschel knew what to do: He had to inform the astronomical community as quickly as possible, to establish his priority as the discoverer. In those days, the members of the worldwide scientific community informed each other of new discoveries, theories, and experiments via a constant blizzard of correspondence, often sending or receiving dozens of letters in a single day. So Herschel, who was only a peripheral member of this informal international network, immediately sent a letter containing the details of his comet to the most senior astronomer he knew: Thomas Hornsby, the director of the observatory in Oxford, with whom he had exchanged a few letters in the past. Through his friend William Watson, who moved in scientific circles in London, Herschel also informed Nevil Maskelyne, at the Royal Greenwich Observatory. Maskelyne was the astronomer royal, the most senior astronomer in the country.

    Maskelyne found the comet almost immediately, and Hornsby found it a few days later. But they both realized that there was something highly unusual about it. "The last three nights I observed stars near the position pointed out by Mr Herschel, whereby I was enabled last night to discern a motion in one of them," Maskelyne wrote to Watson on April 4. But, he added, if this moving star was indeed a comet, it was "very different from any comet I ever read any description of or saw. This seems a comet of a new species." Maskelyne suggested that Herschel write a paper and send it to the Royal Society, the preeminent British scientific society, describing his telescope and his discovery.

    Herschel's comet was unusual because unlike other comets, it had no tail and was not surrounded by a fuzzy cloudlike coma. In fact, it was hardly fuzzy at all. Maskelyne began to suspect that Herschel's comet was, in fact, an entirely new planet.

    Most astronomers were not so sure. Herschel continued to observe what he assumed was a comet, and wrote up his results in a paper that Watson passed to the Royal Society in London, where it was read out loud at the April 26 meeting. Since London was several hours' journey from his home in Bath, Herschel did not attend. Modestly titled "Account of a Comet," his article nonetheless stirred up astonishment and skepticism in equal measure. For although Herschel's account of his discovery was straightforward enough, his casual reference to his eyepieces of 460 and 932 times magnification—along with two others of 1,536 and 2,010 times—astounded the astronomers present. Not even the astronomer royal's telescope, one of the finest available, could magnify any more than 270 times. So Herschel, with his wild claims about the power of his homemade telescope, sounded like a crank.

    Crank or not, there was no denying that Herschel's comet was real and could be seen by any competent astronomer as it made its way through the heavens. Word of the comet soon reached astronomers overseas. Charles Messier, Maskelyne's counterpart in France and a senior figure at the Academy of Sciences (the French equivalent of the Royal Society), wrote to Herschel as soon as he heard of the discovery. A keen comet hunter himself, Messier was particularly impressed by Herschel's ability to spot such a small, faint object.

    As summer approached and the evenings grew lighter, Herschel's comet was lost in the evening twilight and could not be observed again until August. By the time it reappeared in the darkening autumn skies, astronomers had started trying to calculate its orbit.

    To begin with, they based their calculations on the assumption that the orbit was the usual shape for a cometary orbit, a mathematical curve called a parabola. Traveling on a parabolic orbit, a comet swoops in toward the Sun and then hurtles off again into the far reaches of the solar system. But working out a parabolic orbit for Herschel's comet that corresponded to its actual observed motion from night to night proved to be impossible. Even orbits that correctly predicted the comet's motion for a few days quickly became hopelessly inaccurate. Stranger still, the comet did not seem to be getting any larger or brighter, as comets normally do; indeed, Messier noted that with its small disk and whitish light, similar to that of the planet Jupiter, the comet was unlike any of the eighteen comets he had previously observed.

    Anders Lexell, a celebrated mathematician and astronomer from St. Petersburg, Russia, decided to try a different approach. Instead of deriving a parabolic orbit, as would be expected for a comet, he derived the sort of orbit that would be expected of a planet. In 1609 the German astronomer Johannes Kepler had shown that the planets travel around the Sun in almost-circular ellipses. So Lexell performed a calculation to see if the motion of Herschel's comet was consistent with a circular orbit. To his surprise, he found that it was. Furthermore, the orbit was far beyond that of Saturn, the most distant planet from the Sun. Lexell's results, and similar calculations performed soon afterward by other astronomers, tipped the balance of opinion in favor of the idea that Herschel had indeed discovered a planet—one whose faintness, due to its great distance, had prevented anyone from noticing it before.

    This was a truly momentous discovery, and it prompted Sir Joseph Banks, president of the Royal Society, to write to Herschel in November 1781. "Some of our astronomers here incline to the opinion that it is a planet and not a comet," he declared. "If you are of that opinion, it should forthwith be provided with a name." If Herschel failed to move fast, Banks suggested, "our nimble neighbours, the French, will certainly save us the trouble of baptizing it."

    In the same letter Banks also announced that the Council of the Royal Society had decided to award Herschel its highest honor, an annual prize called the Copley Medal, which Herschel was invited to London to receive. At the presentation ceremony on November 30, Sir Joseph made a speech praising Herschel for his discovery of a new planet and for having provided astronomers with a mysterious new body to observe, chart, and scrutinize. He then presented the medal, to great applause.


At the time he made his discovery Herschel was living a double life, combining music with astronomy. His journal entries contain an odd mixture of details of concerts, music lessons, and pupils one minute, and mirrors, glasses, putty, and star maps the next. He was obsessed; every spare moment was devoted to polishing mirrors, building telescopes, and observing the heavens. Often he would return from a concert or a social occasion in Bath and go straight to his telescopes. As his sister Caroline noted in her memoirs, "Every leisure moment was eagerly snatched at for resuming some work which was in progress, without taking time for changing dress, and many a lace ruffle was torn or bespattered by molten pitch."

    Mirror making in particular is not a job for the halfhearted, since the mirror must be continuously polished for hours at a time in order to be free of imperfections. On one occasion, noted Caroline, "by way of keeping him alive I was even obliged to feed him by putting the Vitals by bits into his mouth. This was once the case when at the finishing of a seven-foot mirror. He had not left his hands from it for 16 hours altogether. And in general he was never unemployed at meals, but always at the same time contriving or making drawings of whatever came into his mind. And generally I was obliged to read to him when at some work which required no thinking."

    One of Herschel's pupils, an actor named Bernard, recalled that one evening the sky began to clear in the middle of his music lesson. "There it is at last," cried a jubilant Herschel to the bewilderment of his pupil, dropping his violin and rushing to the telescope to observe a particular star. Bernard also described Herschel's rooms where the lessons took place: "His lodgings resembled an astronomer's much more than a musician's, being heaped up with globes, maps, telescopes, reflectors & c., under which his piano was hid, and the violincello, like a discarded favourite, skulked away in a corner."

    Herschel himself recalled that some of his pupils "made me give them astronomical instead of music lessons." An invariably cheerful and good-tempered man, he was happy to oblige.

    His unexpected discovery catapulted Herschel to international fame. Letters congratulating him were soon coming in from eminent astronomers all over Europe. The French astronomer Joseph-Jérome Lalande wrote from Paris to report that he and his colleagues at the Academy of Sciences, including Messier, had calculated their own approximate circular orbit for the planet and found it orbited the Sun roughly once every 80 years. Since he was writing a book on the history of astronomy, Lalande asked Herschel for information about both himself and his telescope since, he said, astronomers would be "curious about everything which concerns you."

    Many astronomers wanted to know more about Herschel's extraordinarily powerful telescopes. "I congratulate you.... you are the author of a truth which will make your name immortal among Astronomers," wrote the Moravian astronomer Christian Mayer, who went on to ask Herschel whether he would build him a telescope and how much he would charge for it. Similarly, Johann Schörter, a German astronomer based in Lilienthal, wrote to ask if telescopes like the one with which Herschel made his discovery were available for sale; if they were, he said, he would like one, and so would his friend Johann Elert Bode in Berlin. Yet another letter, from Georg Lichtenberg, a German astronomer in Göttingen, praised Herschel as follows: "The accuracy of your observations is hitherto unheard of in astronomy. It has given me special pleasure to see the courage with which you undertake to examine afresh things which we had thought finished and done with."

    But some English astronomers still had their doubts about Herschel. Was he genuinely an astronomer and telescope maker of the highest order or merely a lucky amateur? In December 1781 Watson wrote to Herschel from London to point out just how extravagant the claims he was making about his telescope really were. By this time Herschel had sent a paper to the Royal Society on the subject of double stars, in which he happened to mention the magnification of his most powerful eyepiece, which was over 6,000. This was simply too much for some of the astronomers present. "What! say your opposers," Watson explained, "opticians think it no small matter if they sell a telescope which will magnify 60 or 100 times, and here comes one who pretends to have made some which will magnify above 6000 times! Is this credible?"

    To make matters worse, Herschel had also claimed that many stars that appeared as single stars to other astronomers were revealed by his telescope to be double stars. But nobody else could verify his claims. Was this because their telescopes were inferior, or did Herschel's suffer from some kind of optical flaw? Perhaps, Watson suggested, Herschel should invite other astronomers to examine his telescopes in order to confirm or disprove his claims about their quality. There was even muttering in some quarters that Herschel's planet might yet turn out to be a comet after all.

    Any lingering doubts that Herschel really had discovered a planet were, however, finally demolished in the spring of 1782. By this time a number of astronomers had started working out more accurate orbits for Herschel's planet. The key breakthrough came when Bode discovered that a star, observed by the astronomer Tobias Mayer in 1756 and recorded in his star catalog, had subsequently vanished. The new, more accurate orbits could be used to see where in the sky Herschel's planet would have been at the time. The result was conclusive: Mayer's missing star was exactly where the planet was predicted to have been. Mayer had, without realizing it, seen the planet and mistaken it for a star. Herschel's discovery was thus confirmed beyond all doubt.

(Continues...)

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Meet the Author

Tom Standage is technology editor at The Economist magazine and the author of four history books, "A History of the World in Six Glasses" (2005), "The Turk" (2002), "The Neptune File" (2000) and "The Victorian Internet" (1998), two of which have been serialized as "Book of the Week" on Radio 4. "The Victorian Internet was made into a Channel 4 documentary, "How The Victorians Wired the World". Tom has previously covered science and technology for a number of newspapers and magazines, including The Guardian, The Daily Telegraph, Wired and Prospect. He holds a degree in engineering and computer science from Oxford University, and is the least musical member of a musical family. He is married and lives in Greenwich, London, with his wife and daughter.

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